1. Field of the Invention
The present invention relates to a method for treating a metal surface, and especially to a method for treating a surface of light metals, highly active metals or commonly used metals, such as magnesium, aluminum, zinc, titanium, iron, cobalt, nickel, silver, vanadium and chromium.
2. Description of Related Art
In recent years, due to the shortage of oil energy source, many transportation vehicles and electronic products adopt light metal materials to reduce the weight of the products for requirements for energy-saving and lightweight. For transportation vehicles, lightweight can reduce the fuel consumption required for driving, thus achieving energy conservation. The so-called light metal, generally refers to aluminum, magnesium, zinc, titanium and so on, which has a promising potential for future development due to the low specific gravity and high strength. According to the statistics, the current magnesium production in the world is about 429,000 tons, and is increasing every year. Thus, the applications and needs of a magnesium metal have attracted attentions in various fields. However, magnesium or its alloy has a poor corrosion resistance, and a surface treatment is typically required to enhance the corrosion resistance, thereby increasing its structural stability.
The current methods for treating a light metal surface are mostly anodic and chemical conversion treatments. The anodic or plasma treatment controls the formation of the oxide layer on the metal surface by applying voltage or other energy. However, the formed metal surface is not only rough but also electrically non-conductive, thereby affecting the electromagnetic shielding of the metal and reducing the availability of the metal. The chemical conversion treatment forms a protective layer on the metal surface by a passivator, and a chromate process liquid is typically used. However, chromate compounds are toxic, and their waste liquid is difficult to handle and hazardous to the environment. Therefore, due to the above problems, it is an important object to find a substitute for the chromate process liquid for the chemical conversion treatment.